1. Field of the Invention
The present invention relates to a method and solution for forming or enhancing the properties, such as corrosion resistance, of a conversion coating on metal surfaces or substrates.
2. Background of the Related Art
In general, chemical conversion coatings are formed chemically by causing the surface of the metal to be xe2x80x9cconvertedxe2x80x9d into a tightly adherent coating, where either all or part of the conversion coating consists of an oxidized form of the substrate metal. Chemical conversion coatings can provide high corrosion resistance to the substrate as well as strong bonding affinity for paint. The industrial application of paint to metals generally requires the use of a chemical conversion coating, particularly when the performance demands are high.
Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly aluminum alloys with a high copper content, corrode much more rapidly than pure aluminum.
In general, there are two types of processes for treating aluminum to form a beneficial conversion coating. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed through the aluminum component and the chemical bath. The conversion coating formed on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
The second type of process is by chemically producing a conversion coating, which is commonly referred to as a chemical conversion coating, by subjecting the aluminum component to a chemical solution, such as a chromic acid solution, but without using an electric current in the process. The chemical solution may be applied by immersion application, by manual application, or by spray application. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
Chromate based conversion coatings have been widely used in applications where maximum corrosion protection is an issue. Immersion of aluminum or aluminum alloys in a chromate conversion coating bath results in a thick, corrosion resistant film consisting of hydrated Cr(III) and Al(III) oxides. The reaction is driven by reduction of the high valent Cr(VI) ion and oxidation of the Al metal. Some of the benefits of a chromate conversion coating include hydrophobicity and self-healing properties.
Many aluminum structural parts, as well as Cd plated, Zn plated, Znxe2x80x94Ni plated, and steel parts, throughout the aircraft and aerospace industry are currently being treated using this chromic acid process technology. Chromic acid conversion films, as formed on aluminum substrates, have been shown to meet a 168-hour corrosion resistance criterion, but they primarily serve as a surface substrate for paint adhesion. Because of their relative thinness and low coating weights (40-150 milligrams/ft2), chromic acid conversion coatings do not reduce the fatigue life of the aluminum structure.
However, environmental regulations in the United States, particularly in California, and in other countries are drastically reducing the levels of hexavalent chromium compounds permitted in effluents and emissions from metal finishing processes. Accordingly, chemical conversion coating processes employing hexavalent chromium compounds need to be replaced.
Some of the most investigated non-chromate conversion coatings used in the treatment of aluminum alloy-based materials are described as follows. Sol-Gel technology uses polymers or metal oxides either alone or mixed to form complexes by the hydrolysis of appropriate precursor compounds. Sol-Gels can form powders or thin films that inhibit corrosion on substrates.
Fluorozirconium coating technology uses complexed transition metal salts to create a thin film on a substrate material similar to a conversion coating. Specifically, zirconium is mixed with fluorine to create fluorozirconium, which reacts with the part surface to form a coating.
Cobalt-based coatings use cobalt and molybdenum to treat substrate materials. The coatings created are low in electrical resistance and are good for corrosion resistance.
Rare Earth Metal (REM) salts may be applied by heated immersion to create protective layers on substrate materials. REMs provide corrosion resistance by producing a protective oxide film.
Potassium permanganate solutions can be used to create manganese oxide films on substrates. Manganese oxide films resulting from potassium permanganate treatment closely match the corrosion resistance of traditional chromic oxide films used in conversion coatings. Potassium permanganate coatings can be very effective in protecting aluminum alloys.
Fluotitanic coatings, deposited from acid solutions with organic polymers, require few process steps, and can usually be done at ambient temperatures. Although these coatings have been widely used in a variety of applications, they have not been used in the aerospace industry.
Talc coatings, which are typically applied to aluminum substrates, are resistant to corrosion. These polycrystalline coatings are applied by precipitating aluminum-lithium compounds and other anions in an alkaline salt solution.
Anodizing is a process in which a metal surface is converted to an oxide layer, producing a tough, adherent surface layer. A thick oxide layer can be produced by immersing a part in an electrolytic solution and passing an electrical current through it, similar to electroplating. Then, by placing the part in boiling water, the film""s pores can be sealed. As a result, the oxide changes from one form to another.
Despite these alternatives, there is a continuing need for a conversion coating solution that will form a stable, corrosion-resistant conversion coating on metal surfaces without containing or producing toxic chemicals. Additionally, it would be desirable if the conversion coating provided a suitable surface for receiving organic coatings or paints.
The present invention provides a method for treating a conversion coating on a metal surface, comprising controlling the concentration of carbon dioxide in an aqueous solution, then combining calcium hydroxide with the aqueous solution to form an aqueous calcium hydroxide solution, and then providing contact between the conversion coating and the aqueous calcium hydroxide solution. The step of controlling the concentration of carbon dioxide in the aqueous solution may comprise heating the aqueous solution, passing the aqueous solution through an electroosmotic pump, sparging the solution using and inert gas, or any other known technique. The aqueous calcium hydroxide solution thus formed may be applied to the conversion coating by submersing, spraying, brushing or combinations thereof. The aqueous solution may contain water from any source, including tap water, deionized water, distilled water, sterilized water and combinations thereof. The aqueous calcium hydroxide solution has a calcium hydroxide concentration up to saturation, preferably between about 0.015 and about 0.15% by weight, and more preferably between about 0.06 and about 0.09% by weight.
The calcium hydroxide solution may be used as a post-treatment on conversion coatings formed using any conversion coating solution, such as those having one or more oxidants selected from permanganates, molybdates, polyoxometalates, heteropolyoxometalates, ferrates, cerium compounds, alkaline solutions of lithium salts, zirconates, and combinations thereof. The preferred conditions for treating the conversion coating include contacting the conversion coating with the aqueous calcium hydroxide solution for between about 1 and about 20 minutes at a temperature between about 25 and about 100xc2x0 C. Optionally, it may be desirable to control exposure of the aqueous calcium hydroxide solution to carbon dioxide, such as by providing an inert gas atmosphere over the aqueous calcium hydroxide solution.
Alternatively, the invention provides a method for treating a conversion coating, comprising obtaining an aqueous solution having a carbon dioxide concentration is below about 0.855 grams per liter and most preferably below about 0.454 grams per liter, then combining calcium hydroxide with the aqueous solution to form an aqueous calcium hydroxide solution; and then providing contact between the conversion coating and the aqueous calcium hydroxide solution.
The invention also provides a method for treating a metal surface, comprising controlling the concentration of carbon dioxide in an aqueous solution; then combining calcium hydroxide with the aqueous solution to form an aqueous calcium hydroxide solution; and then contacting the metal surface with the aqueous calcium hydroxide solution.
The method may be used to form a stable conversion coating on a metal substrate, comprising: forming a conversion coating using a conversion coating solution having one or more oxidants selected from permanganates, molybdates, polyoxometalates, heteropolyoxometalates, ferrates, cerium compounds, alkaline solutions of lithium salts, zirconates, and combinations thereof; and treating the conversion coating with an aqueous calcium hydroxide solution prepared by combining calcium hydroxide with an aqueous solution having less than 1.73 grams of carbon dioxide per liter. Preferably, the aqueous solution has a carbon dioxide concentration less than 0.85 grams per liter, and more preferably less than 0.45 grams per liter. The conversion coating may include, but is not limited to, a boehmite layer, preferably formed on the metal surface by anodization or boiling in water prior to use of the conversion coating solution.
The invention also provides a method for preparing a conversion coating solution, comprising controlling the concentration of carbon dioxide in an aqueous solution, then combining calcium hydroxide with the aqueous solution to form an aqueous calcium hydroxide solution. The step of controlling the concentration of carbon dioxide in the aqueous solution may comprises either heating the aqueous solution, such as to a temperature between 50 and 100xc2x0 C., passing the aqueous solution through an electroosmotic pump, or sparging the solution using and inert gas.